This invention relates to electronic devices, and more particularly to a packaged power semiconductor device.
Electronic semiconductor device technology continues to rapidly advance, producing devices that achieve heretofore unattainable operating speeds, packing density, and high-temperature compatibility. This last characteristic, high-temperature compatibility, is one of more important features. High-temperature environments as well as high-power and high frequency applications are becoming increasingly important. The ability of electronics to withstand high-temperature environments such as gas-turbines, automotive engine compartments, materials handling and fabrication equipment, and aircraft subsystems makes possible many historically impractical electronic applications for these environments.
Many such high-power, high-temperature electronics applications inherently limit the ability to dissipate heat and power generated by the electronics. Thermal and power management as well as environmental protection of the electronics are therefore handled entirely by the electronics packaging. As a result, typical high-performance packaging criteria include mechanical strength, high thermal conductivity, close matching of device and package thermal expansion coefficients, low dielectric constant for accommodation of high frequency devices, and environmental protection, such as hermetic sealing.
In designing such packages, thermal management is important—especially at high power generation levels which increase the amount of heat created. Heat decreases the performance and operating life of the transistor. Moreover, heat generated at the collector-base junction can create a temperature buildup which may eventually cause the device to short-circuit.
The power device may be mounted on a thermally conductive pad which acts as a heat sink and dissipates the heat generated by the power device. In order to prevent electrical losses, however, the thermally conductive pad provided to be electrically insulating. Hence, a thermally conducting, electrically insulating material, e.g., beryllia (beryllium oxide—BeO) and alumina (aluminum oxide), is used for the mounting pad.
FIG. 1 illustrates a conventional packaged power semiconductor device 10. The device 10 includes a semiconductor chip 11 attached to a base plate or substrate 14, with a solder layer 13, which is usually a highly thermal conductivity metal, e.g., copper or aluminum. The side of the chip 11 that is attached to the metal base plate is generally referred to as a back side of the chip. The top surface of the chip is connected to the leads 16 of the package by wire bonds or clips 12 as shown in FIG. 1.
The top side of the chip has electrical connections and is covered with plastics in a plastic molded package. The combination of the electrical connections and the plastic package provide minimal thermal cooling of the power chip during its operation. Accordingly, the main cooling path for the chip is from its backside via the base plate to an external heatsink 15 attached thereto. This primary heat dissipation path is shown by an arrow H1 in FIG. 1.